Ceramic composite and method

ABSTRACT

A ceramic composition and method for making ceramic composites having superior erosion and corrosion resistance to molten metal. The composite includes mullite, aluminum nitride and boron nitride in combination. The composite may be hot pressed or cold formed and pressureless sintered into a desired shape.

FIELD OF THE INVENTION

This invention relates to a ceramic composite and to a method of making ceramic composites having superior erosion and corrosion resistance to molten metal.

BACKGROUND OF THE INVENTION

In the continuous casting of molten steel a break ring is used to provide a thermal barrier at the interface between the furnace nozzle and the mold. The break ring must possess thermal shock resistance, stability at high temperature and must be corrosion and erosion resistant to the high temperature molten steel which flows through the break ring into the mold. The ability to resist attack from the molten metal passing through the break ring as it flows into the extrusion mold determines the tonnage of metal that can be cast in a single operation without interruption of the process to replace the break ring. Cost is another important factor which requires the break ring to be machinable.

Boron nitride is a conventional material used in the fabrication of break rings. It is desirable because of its good thermal shock resistance, stability at high temperature and machinability. However, it lacks good abrasion resistance which subjects it to high wear rates when exposed to flowing molten metal. Boron nitride has also been combined to form a ceramic composite with alumina (Al₂ O₃) which is also used in molten metal applications due to its hardness, abrasion resistance and chemical stability. In addition, boron nitride has been separately combined with aluminum nitride (AlN), titanium diboride(TiB₂), mullite(3Al₂ O₃ -2SiO₂) and with aluminum nitride and titanium diboride. Other materials have also been combined with boron nitride such as silicon nitride to form a composite for use as a break ring. However a silicon nitride composite is not readily machinable. The boron nitride composites BN-AlN, BN-3Al₂ O₃ -2SiO₂, and BN-TiB₂ -AlN are readily machinable and are commercially available from the Praxair Inc., advanced ceramics division, located in Cleveland Ohio. The mechanical and physical properties of Al₂ O₃ -BN and mullite-BN composites are described in Lewis et al in "Microstructure and Thermomechanical Properties in Alumina and Mullite Boron Nitride Particulate Ceramic-Ceramic Composites", Ceram. Eng. Sci. Proc. 2: 719-727 (Nos.7-8,1981) which also includes data on the thermal shock resistance of such composites. In addition, U.S. Pat. No. 4,997,605 discloses a hot pressed ceramic composite formed from a blend of fused zirconia mullite and boron nitride which is indicated as having good resistance to thermal shock and reasonably good erosion and corrosion resistance to metal alloys.

The above identified ceramic composites of boron nitride which are readily machinable are all currently formed by hot pressing and have substantially similar corrosion and erosion resistant properties under test conditions which simulate the process conditions of a continuous casting operation. The composite formulation of any of the known boron nitride composites may be adjusted to increase its corrosion and erosion resistance but only as a tradeoff against other properties particularly machinability.

SUMMARY OF INVENTION

In accordance with the present invention it has been discovered that a ceramic composite containing boron nitride, mullite and aluminum nitride, in combination, yields a substantial increase in corrosion and erosion resistance to attack from molten metals which is completely unexpected in comparison to the corrosion and erosion resistance of the known boron nitride composites BN-AlN and BN-3Al₂ O₃ -2SiO₂ as well as that of boron nitride alone. It was further discovered in accordance with the present invention that the a composite containing boron nitride, mullite and aluminum nitride, in combination, is readily machinable and may be fabricated by either hot pressing or cold forming. When formulated in accordance with the present invention the ceramic composite of BN-AlN-3Al₂ O₃ -2SiO₂ has a high resistance to thermal shock and provides superior corrosion/erosion resistance to molten metals relative to the corrosion/erosion resistance of all presently known readily machinable boron nitride composites under similar operating conditions. When the boron nitride composite of the present invention is hot pressed its corrosion and erosion resistance corresponds to a reduction in mass of less than one (1) percent per hour under simulated continuous casting operating conditions. Alternatively, if the ceramic composite BN-AlN-3Al₂ O₃ -2SiO₂ is cold formed in accordance with the present invention its corrosion/erosion resistant properties are as good, comparitively, to the corrosion/erosion properties of known readily machinable hot pressed boron nitride composites.

The ceramic composite of the present invention comprises, in combination, 18.5 to 29.0 weight percent mullite, 35 to 18 weight percent aluminum nitride, balance boron nitride in a minimum concentration of at least about 40 percent by weight.

The present invention is also directed to a method for cold forming a ceramic composite comprising the steps of:

(a) blending a mixture comprising from 18.5 to 29.0 wt % mullite, 35 to 18 wt % aluminum nitride and at least 40 wt % boron nitride;

(b) milling the blend to an average particle size of less than about 5 microns;

(c) coating the particles with a resinous lubricant composed of a vinyl chloride-vinyl acetate resin dissolved in an organic solvent;

(d) compressing the particles into a cold formed shape; and

(e) pressureless sintering the cold formed shape.

Another aspect of the present invention is a method for producing a hot pressed boron nitride ceramic composite comprising the steps of:

(a) blending a mixture comprising from 18.5 to 29 wt % mullite, 35 to 18 wt % aluminum nitride and at least 40 wt % boron nitride;

(b) heating and compressing the blended mixture in a mold at a temperature of between 1650° C. and 1900° C. and at a pressure of between 1800 and 2500 psi; and

(c) cooling the composite at a gradually decreasing pressure so that fracture of the composite is prevented.

DETAILED DESCRIPTION OF THE INVENTION

The ceramic of the present invention is composed of a composite formed from a mixture of the ceramic materials boron nitride(BN), mullite(3Al₂ O₃ -2SiO₂), and aluminum nitride (AlN). A densification aid such as CaO should preferably be added to the mixture. Mullite is an orthorhombic homogeneous solid solution of alumina in sillimanite and is commercially available in powder form having an average particle size of less than 10 microns. Boron nitride and aluminum nitride are also commercially available in powder form. Each of the materials in the composite may vary in the following proportion by weight:

    ______________________________________                                         Material     Maximum Range                                                                               Preferred Range                                      ______________________________________                                         CaO          1% to 5%     3.0% to 2.0%                                         AlN          18% to 35%   18.5% to 29%                                         3Al.sub.2 O.sub.3 --2SiO.sub.2                                                              30% to 18%   18.5% to 29%                                         BN           balance      60% to 40%                                           ______________________________________                                    

In order to form a homogeneous mixture a portion of the powders should first be preblended in a mixer such as a V-blender using preferably all of the CaO. The preblend should preferably constitute 25% of the total mixture. The other 75% of the preblended mixture should preferably constitute relatively equal amounts of BN,3Al₂ O₃ -2SiO₂, and AlN. The preblend should then be mixed for at least 30 minutes preferbly with an intensifier bar. After the preblend is prepared the V-blender should then be loaded to produce the main blend for the composite in a systematic fashion preferably by layering the V-blender with successive layers of material as is well known to those skilled in the art. The preferred loading of the V-blender in the preparation of the main blend may be carried out as follows:

1. Add substantially 1/2 of the balance of BN.

2. Add substantially 1/6 of the preblend.

3. Add 1/2 of the balance of AlN.

4. Add 1/6 preblend.

5. Add 1/2 balance of 3Al₂ O₃ -2SiO₂.

6. Add 1/6 preblend.

7. Repeat steps 1-6.

8. The V-blender should be operated for at least 60 minutes preferably with an intensifier bar.

To hot press the material the main powder blend may be added directly to a graphite mold for densification. However, the main powder blend should at first preferably be prepressed into compacts and granulated before being added to the graphite mold. Densification by hot-pressing takes place as a function of temperature and pressure. Hot pressing may be carried out at a temperature of about 1800° C. and a pressure of 2200 psi with a hold time at maximum conditions for two hours. The temperature should rise at a heat rate of typically about 300° C./hour and pressure should be applied gradually reaching full pressure at a temperature of about 1000° C. Pressure can be retained at maximum during cooling to about 1000° C. and should then drop gradually with further cooling. The raw material powders used for both hot pressing and cold forming should have a purity of at least 95% with the boron nitride, mullite and calcium oxide purity of preferably over 97% and with the boron nitride and aluminum nitride of low oxygen concentration. The boron nitride particles should have an average size of less than 1 micron with the other components of the mixture having a particle size of less than 10 microns. Typical properties for the hot pressed composite both with and without calcium oxide is shown in the following two Tables I and II respectively:

                  TABLE I                                                          ______________________________________                                         A Few Properties of Hot-Pressed BN--AlN--3Al.sub.2 O.sub.3.2SiO.sub.2          Composition:                                                                               55.64% BN-20.93% AIN - 20.93%                                                  3Al.sub.2 O.sub.3.2SiO.sub.2 + 2.5% CaO                            Fabrication:                                                                               Hot-Pressed 1800° C.-2200 psi, two-hour                                 hold, pressure decayed gradually to 0                                          at ˜1600° C. and billet ejected into                              mold taper at 1100° C.                                      ______________________________________                                                         Test Sample Direction                                                          to Hot-Pressing                                                Properties        Parallel Perpendicular                                       ______________________________________                                         Density, % Theoretical                                                                           92.2     92.2                                                Flexure, psi:                                                                  25° C.     10,400   15,600                                              1500° C.    4,400    6,500                                              Sonic Modulus, psi ×                                                                       5.0      8.0                                                 10.sup.6 : 25° C.                                                       CTE, in./inc/°C. × 10.sup.-6                                                        5.2      5.6                                                 Thermal Conductivity,                                                                            11.0     14.0                                                Watts/M °K.                                                             ______________________________________                                         Erosion/Corrosion.sup.(1) in Liquid Metals                                     as Percent Reduction in Diameter of                                            Test Sample per Hour:                                                                                Test Sample Percent                                      Metal          °C.                                                                            Reduction Per Hour                                       ______________________________________                                         304 Stainless Steel                                                                           1535   0.70                                                     Low-Carbon Steel                                                                              1550   0.17                                                     Gray Cast Iron 1480   0                                                        Ductile Cast Iron                                                                             1480   0                                                        ______________________________________                                          .sup.(1) Test sample 1/2 in. diameter × 2.0 in. rotating at 60 rpm       for exposure time of four hours                                          

                  TABLE II                                                         ______________________________________                                         A Few Properties of Hot-Pressed BN--AlN--3Al.sub.2 O.sub.3.2SiO.sub.2          (No CaO) Composition: 57.06% BN- 21.47% AlN- 21.47% Mullite                                    Test Sample Direction to Hot-                                                  Pressing                                                                       Parallel                                                                              Perpendicular                                           ______________________________________                                         Density, g/cc      1.70     1.69                                               % Th.             66.93    66.54                                               % Porosity        33.07    33.46                                               Flexure Strength, psi                                                          25° C.     1598     1820                                                1500° C.   1874     2130                                                Sonic Modulus, psi × 10.sup.6                                                               0.83     1.08                                               ______________________________________                                         Erosion/Corrosion, Percent                                                     Reduction in Diameter of                                                       Test Sample per Hour.sup.(1)                                                   % Reduction   Metal                                                            ______________________________________                                         1.66          L.C. Steel (1500° C.)                                     0.27          304 S.S. (1535° C.)                                       0.83          Grey Cast Iron                                                                 (1480° C.)                                                ______________________________________                                          .sup.(1) 1/2 In. Diameter × 2 In. Long Sample, 60 RPM for four hour                                                                               

A comparison of the corrosion/erosion rate of the composite of the present invention to the corrosion/erosion rate of boron nitride and composites of boron nitride-aluminum nitride and boron nitride-mullite is shown in the following table III:

                  TABLE III                                                        ______________________________________                                         Comparison of the Erosion/Corrosion Rate of Commercial BN                      Composites in Selected Metal Compared to BN Invention Composite                            Grades Hot-Pressed                                                                                       Invention                                Component     HBR      ALN-60   MBN   MBA-Z                                    ______________________________________                                         BN            98.0     56.0     50.0  55.64                                    AlN           --       38.0     --    20.93                                    3Al.sub.2 O.sub.3.2SiO.sub.2                                                                 --       --       50.0  20.93                                    CaO           --       6.0      --    2.5                                      CaF.sub.2     2.0      --       --    --                                       Errosion/Corrosion in                                                          Molten Metals as Per-                                                          cent Reduction in                                                              Diameter of Test Sam-                                                          ple (1/2 In. Diameter ×                                                  2 In. Long) per Hour                                                           When Samples are Ex-                                                           posed to Molten Metals                                                         at 60 RPM for                                                                  Four Hours:                                                                    °C.                                                                          Metals                                                                    1535 304 Stainless Steel                                                                         ˜35.0                                                                             2.30   3.20  0.70                                   1550 Low-Carbon   ˜15.0                                                                             2.10   2.15  0.14                                        Steel                                                                     1480 Grey Cast Iron                                                                              0.167    --     0.138 0                                      1480 Ductile Cast Iron                                                                           0.50     --     0     0                                      ______________________________________                                    

The boron nitride composite of the present invention can be cold formed and pressureless sintered to result in a strong thermally stable stock that offers excellent resistance to molten metals. The preferred cold forming process involves the following steps:

a. Milling the V-blended main blend, which is preferably blended as indicated earlier in connection with hot pressing, to a predetermined average particle size of 3-5 microns (microtrac).

b. Coating the particles with a lubricant of a resin material formed from vinyl chloride and acetate.

c. Cold-forming the particles into the desired shape.

d. Pressureless sintering the cold-pressed article under an inert atmosphere at a temperature of between about 1800° C. and 1975° C., preferably about 1940° C., to produce a strong structure.

The above identified milling step is preferably wet milled in an organic solvent in which the vinyl chloride and vinyl acetate/polyvinyl acetate is dissolved. The preferred solvent is a ketone such as methylethylketone (MEK). By wet milling in a solvent the particles will automatically be coated as indicated above in step "b". Alternatively, the coating could be put on by spray-drying methods. The wet milled powder is dried and crushed to size and screened to an agglomerated size of between -65+325 mesh. The cold forming step "c" can be conducted at pressures such as 35,000 psi in well lubricated metal dies, isostatically molded or slip cast as is well known to those skilled in the art. The cold formed article has a green density of about 55 to 60% theoretical which is substantially uniform throughout its volume. Moreover, if the cold-formed shape is cured at a temperature of about 120° to 160° C., preferably at about 150° C., machining of the product can be readily carried out.

To avoid the formation of cracks in the cold formed product during the step of pressureless sintering the heating of the product should be uniform. This may be accomplished by enclosing but not confining the green shape in a graphite container to permit the formed shape to freely expand or shrink during the heating cycle. Heat rate depends upon cross sectional size of the product but should not exceed about 400° C./hour, preferably about 200° C./hour to about 1900° C. with no more than a 10% temperature drift for about a holding period of 3 hours. The cold formed articles of BN-AlN-3Al₂ O₃ -2SiO₂ -CaO with a composition of 55.64% BN -20.93% APN - 20.93% mullite - 2.5% CaO bond during pressureless sintering to produce strong machinable stock with densities in excess of 70% of theoretical, and the structure is not substantially wet by liquid 304 stainless steel or low carbon steel. For example, the reduction in the diameter of test samples (1/2 in. diameter×2 in. long) per hour rotating in liquid metal at 60 rpm for four hours are shown below.

    ______________________________________                                                     Typical Range                                                                  % Reduction                                                                              Temperature °C.                                   ______________________________________                                         304 Stainless Steel                                                                          1.6-1.8     1535                                                 Low-Carbon Steel                                                                             1.6-2.1     1550                                                 ______________________________________                                     

What we claim is:
 1. A method for cold forming a ceramic composite into a machinable article having good corrosion/erosion resistance to molten metal comprising the steps of:(a) blending a mixture comprising from 18.5 to 29 wt % mullite, 35 to 18 wt % aluminum nitride, balance boron nitride in a minimum concentration of at least about 40 percent by weight; (b) milling the blend to an average particle size of less than about 5 microns; (c) coating the particles with a resinous lubricant composed of a vinyl chloride-vinyl acetate resin dissolved in an organic solvent; (d) compressing the particles into a cold formed shape; and (e) pressureless sintering the cold formed shape.
 2. A method as defined in claim 1 wherein the blended mixture includes between 1 to 5 wt % calcium oxide.
 3. A method as defined in claim 2 wherein the blend is milled to an average particle size of between 3 to 5 microns.
 4. A method as defined in claim 3 wherein a selected proportion of the particles are first preblended with substantially all of said calcium oxide with said preblend blended with substantially equal amounts of each of the other components in the mixture to form a total preblend.
 5. A method as defined in claim 3 wherein said organic solvent is methyethylketone.
 6. A method as defined in claim 5 wherein said particles are compressed into a cold formed shape at between 55 to 65% of theoretical density.
 7. A method as defined in claim 6 wherein said cold formed shape is pressureless sintered at a uniform temperature with a heat rate of less than about 400° C./hour.
 8. A method for producing a hot pressed boron nitride ceramic composite comprising the steps of:(a) blending a mixture comprising from 18.5 to 29 wt % mullite, 35 to 18 wt % aluminum nitride and at least 40 wt % boron nitride; (b) heating and compressing the blended mixture in a mold at a temperature of between 1650° C. and 1900° C. and at a pressure of between 1800 and 2500 psi; and (c) cooling the composite at a gradually decreasing pressure so that fracture of the composite is prevented.
 9. A method as defined in claim 8 wherein the blended mixture includes between 1 to 5 wt % calcium oxide. 